This can be completely false and made up but I thought it was weird enough to report here. A DigitalPoint Forum thread named Mysterious GMail Image - Girl Sticking Out Tongue! shows a link to a comment at a particular blog, where a gmail user says his gmail logo was replaced by a picture of a woman. He posted a screen shot here of the gmail screen, with the woman replacing the logo.
This site also notes the image, saying;
Have You Seen This Person?Many Gmail users have seen this image when checking their mail tonite. This journalist figures it must have been an inside job because Google has a whole team of security experts. The question is then, when will we know for sure?
I am not sure what exactly is up here, it can most likely be a hoax. Or perhaps Google getting ready for April Fools Day.
Forum discussion at DigitalPoint Forums.

Comments:
Chris
03/30/2006 01:24 pm
This is probably a screenshot from the Beta for the Gmail Domains service, where you can put any logo you want in that spot.
Drew
03/30/2006 04:07 pm
Well, my girlfriends sister has a xanga and there is some spam thing on there (of course) and that image comes up and says something like "come see my cam" ...whatever. It could very well be a cache problem if some of you have visited any part of the xanga website.
troy
03/30/2006 09:15 pm
I was the person that actually discovered this error and reported it late last night. I had made a post in google groups and it looks like the gmail team took care of it. I still would like to know the story behind the image, if google's security is really up to par, especially with emails from millions of users.
troy
03/30/2006 09:35 pm
Most definately not a cache problem, I dumped my cache in firefox, and viewed in IE, restarted the machine, the whole bit. I even viewed the image and saved it to my server, the quality is low. Just click my name on this comment to bring you to the URI
Melinda Chiu
03/31/2006 07:47 pm
Well, it might be another great works of Adobe or April Fool, Maybe.